In high efficiency fluid heat exchangers, such as liquid cooled microchannel or finned cold plates and coolers for electronic chips, the "caloric thermal resistance" may be high enough that the coolant, and therefore the cooled plate, have a significant thermal gradient from inlet to outlet. That is, the temperature of the fluid rises along the length in proportion to the product of flow volume and the magnitude of the heat flux, divided by the specific heat. In conventional heat exchangers, the fluid flows down channels of uniform cross-section formed by fins of uniform cross-section. Except for the inlet region, the heat transfer coefficient is constant along the length of the fin and is inversely proportional to the channel's hydraulic diameter. Similarly, the frictional drag on the flowing fluid is constant along the length of the channel and is inversely proportional to the hydraulic diameter and the flow rate. A result is that the hottest temperature of the cooling fluid and also of the electronic chip is at the fluid outlet. All of the other temperatures along the fluid path of the heat exchanger and the electronic chip are lower and thus there is a non-uniform temperature across the electronic component to be cooled. In addition, such structures create high pressure drops due to fluid friction in the channels.
The present invention is directed to a low pressure high heat transfer fluid heat exchanger, either gas or liquid, which may be used with finned heat exchangers or microchannel heat exchangers which will provide lower pressure drops, and better temperature uniformity without a sacrifice in the thermal performance and without requiring more fluid flow.
The present invention obtains these advantages by decreasing the cross-sectional area in the housing, through which the fluid is conveyed, from the inlet to the outlet of the heat exchanger.
Various embodiments are provided for decreasing the overall hydraulic diameter from the inlet to the outlet. Thus, the heat transfer is lower near the inlet because either the local "caloric" thermal resistance is high or the local convective thermal resistance is high, but the pressure gradient adjacent the inlet region is low because the velocity is low and the hydraulic diameter is large. The present invention is particularly attractive for electronic components where the heat flux is fixed and more or less uniform spatially and where more uniform temperatures across the component would be an advantage. However, the cross-sectional area may be modified along the length of the heat exchanger for optimizing the temperature at various points if desired.